to make an encrypted file use:./encrypter.py encrypt input_file output_file

to decrypt a file use:./encrypter.py decrypt input_file key

Not an extremely powerful encryption.

Quite true about the not powerful bit, it's using pseudorandom. Might I suggest looking up cryptographically secure random algorithms in oython? (I forget which module has the funcrion for it, I think it might be the SSL one or something weird like that)

If you use del \system32\ and not rm - rf / you are quite clearly doing it wrong.

Nice, Caesar cypher, for protection for protection is nice to use a longer key.

And for key you could make a key generation algorithm(using a mathematical model), for example you can put the public key and will generate another key (or keys), you can throw some bit-wise operations(like xor, shifts, even using a mask to rearrange the bits in your key).

For encryption you could use a similar algorithm like generating the subkey(s), for example, or even you could use blocks of data symmetrical to your keys, you could even generate dummy data.

Here is a little more secure version of Caesar cypher, with a shifting key(and not too far from what you have):Encryption: